Method of fabricating electron tubes having photocathodes



July 20, 1 965 A. H. SOMMER 3,195,972

METHOD OF FABRICATING ELECTRON TUBES HAVING PHOTOCATHODES Filed Sept. 21, 1962 N: R N 3 N N 85 I 'f 3' I m3 j 'l U- j 1 s w I iqllll fllllllllf lvl United States Patent 3,195,972 METHOD OF FABRHCATING ELEKITRON TUBES HAVING PHOTOCATHODES Alfred H. Sornrner, Princeton, N.J., assiguor to Radio Corporation of America, a corporation of Delaware Filed Sept. 21, 1%2, Ser. No. 225,274 9 Claims. (Cl. 3ll6-6) The present invention relates to electron tubes having photocathodes and particularly to a method of fabricating a photocathode in which the photocathode in nearly completed form, can be exposed to air without adverse effects on the sensitivity of the completed photocathode.

One type of photocathode employed widely is a semitransparent type. This type includes a transparent substrate, having thereon a coating that responds in electron emission to light impinging thereon. This type of cathode finds utility in a variety of conversion type electron tubes such as camera tubes, image tubes, photomultipliers and others. 7 In one form of this type of photocathode, relatively thin layers of silver and bismuth are evaporated successively on a flat glass substrate. The resulting composite layer is then superficially oxidized and activated with cesium. One problem associated with this method of making a silver-bismuth-oxygen-cesium photocathode resides in the need to form the coatings after evacuation of a tube in which the photocathode is used, since it was found that the silver-bismuth layer could not be exposed to air without adverse effects on the sensitivity of the completed photocathode. While this technique of forming the coatings after tube evacuation has been used in connection with tubes having elongated envelopes, it presents difliculties in connection with relatively short tubes.

These difficulties arise for short tubes because uniformly thick silver and bismuth layers require an appreciable distance between the substrate being coated and a point source of the coating materials during a coating operation. Such uniformity in thickness could ideally be achieved if the surface to be coated constituted a portion of a sphere and the evaporator or source of coating material were located at the center of the sphere. Where the length of the tube permits an appreciable distance between the substrate to be coated and a coating material evaporator positioned therein, a practical close approximation to the ideal condition mentioned can be realized, even though the substrate is flat. Such tube length, however, is not available in many tube types in which it is desired to use a silver-bismuth-oxygen-cesium photocathode. For example, some types in which this photocathode could be used to advantage except for the foregoing difficulty, are relatively short image tubes and flat photoelectric energy converters.

Furthermore, it may be desirable even in relatively long tubes having photocathodes, to form the major portion of the photocathode coating such as the silver, bismuth and oxygen, prior to evacuation of the tube. This possesses the advantages of permitting application of the silver and bismuth coatings on a substrate and oxidation of the bismuth, in a region remote from the remainder of the eventual tube envelope and of simplifying tube design and optics. In this way the evaporator and the substrate may be mutually spaced even farther than the maximum spacing tolerated by the tube length, to thereby achieve an even closer approximation to the ideal coating condition aforementioned. Moreover, the formation of the photocathode prior to tube evacuation aifords a desirable ease of handling and manufacturing convenience.

Accordingly, it is an object of the invention to provide an improved method of making an electron tube having a photocathode.

3,l95,9-7Z Patented July 20, 1965 It is another object to provide an improved method of making a photocathode including silver and bismuth.

A further object of the invention is to provide a partly processed photocathode including bismuth, silver and oxygen, that is free from harmful effects when exposed to air for an appreciable time period.

One type of electron tube in which the invention may be practiced advantageously, includes an envelope having a glass portion serving as the substrate for a photocathode. Image tubes and Image Orthicon tubes are of this variety. The invention also finds utility in electron tubes wherein the substrate for a photocathode has a structure that is independent of a tube envelope.

An example of the invention in connection with a tube in which a portion of the glass envelope thereof serves as a substrate, for a silver-bismuth-oxygen-cesium photocathode, involves fabrication of the envelope in two parts. One of the parts comprises the portion of the envelope intended for service as the photocathode substrate. The isolation of this envelope portion from the remainder of the envelope permits the application thereto of some of the elements of a photosensitive coating to wit, silver, bismuth and oxygen, under conditions that constitute a close approach to the ideal with respect to uniformity in thickness of the coating. The application of a portion of the photoemissive coating to the substrate in a region remote from the space defined by the interior of the eventual tube envelope is rendered feasible because of an important discovery by applicant. Applicant has discovered that if the bismuth layer of the coating is completely oxidized after application, the coated substrate can be stored in air for appreciably long time periods without harmful effects. In one example, this time period was 36 hours. Applicant found that a coated substrate, fully oxidized as indicated, could be activated in the normal way with cesium after exposure to air. The resultant photocathode had sensitivities equal to photocathodes made in accordance with prior practices, which involved only superficially oxidizing the bismuth in the bismuth-silver layers without exposure to air.

Further objects and features of the invention will become apparent from a consideration of illustrative embodirnents taken in connection with the accompanying drawing wherein:

FTG. 1 is a sectional view of a portion of an image tube envelope in operative relation with respect to a coating applicator;

PEG. 2 is a sectional View of an image tube in which the envelope thereof includes the aforementioned envelope portion;

FIG. 3 is a sectional view of a portion of the envelope of an Image Orthicon tube in desirable spaced relation with respect to a coating applicator; and

FIG. 4 shows an elevation, partly in section, of an Image Orthicon tube including the portion of the envelope shown in FIG. 3.

A more detailed consideration of the drawing will reveal that FIG. 1 thereof shows a dish-shaped subassembly 1t) forming part of the envelope of the image tube shown in FIG. 2. The subassernbly 10 in the example shown is made of glass and includes a flat face plate 12 and a cylindrical flange 14 integral with the faceplate 12.

An evaporator 16 is positioned to direct coating material to the inner surface of faceplate 12. The evaporator comprises a coil 18 of wire, made of tungsten for example, and supporting a pellet 29 of coating material. The coil 13 is adapted to be connected to a suitable electrical current supply, and to be heated by current losses therein, to thereby heat the pellet 20 to a temperature at which it vaporizes and flashes outwardly of the coil 18. The inner surface of the faceplate 12 intercepts desired position with respect to the surface to be coated.

Such desired position is determined by several factors controlling the uniformity in thickness of the 'ap-. plied coatings.

the evaporator 16; This spacing is related to the diameter of the faceplate 12, and should be at least as large as the diameter of the faceplate to assure a high degree of thickness uniformity in the applied coating. As can be seen in FIG. 1', the edge portion 26 of the flash or spray, traverses a longer path than the central portion 28 of the spray. Consequently in a given interval of time more coating material will accumulate on the cen-' sirable, however, to form the faceplate 12 to spherical- ;contour, and coating thickness uniformity obtained by the aforementioned close approximation to the ideal con-l dition for such uniformity is accomplished by satisfactory results. always be obtained where the coating material must be applied from a source within a tube envelope after evacuation of the tube. This is either because the tube length is inadequate to. provide 'the required spacing, or,

the tube structure restricts the location of the evaporator so that the full length of the tube cannot be availed of for securingthe desired spacing.

In the example shown in FIG. inner surface of the faceplate 12 wasabout 3 inches,

and the spacing between such inner surface and the evaporator 16 was about 3 inches. At this distance, a satisfactory uniformity in thickness of the applied coating was obtained.

In coatingthe inner surface of'the faceplate 12 first with a silver coating 22, the pellet 20 was made of silver. The coil 18 was connected to a suitable electrical current supply for heating the silver pellet to a temperature at which it vaporized. ,The inner'surface of faceplate 12 was exposed to the vaporized material until the light transmission through the applied silver coating .22 was reduced to approximately 80% of the light transmission through the uncoated faceplate. 1 V

Thereafter an evaporator having a pellet 20 made of bismuth was placed in operative position instead of the evaporator having the silver pellet, and suitably heated to. cause the bismuth pellet to vaporize. The inner silver coated surface of the faceplate 12 was exposed to this vapor until light transmission through the doubly coated faceplate fell to approximately 5%'of thelight transmission through the bare faceplate. V

The foregoing coating applications were both performed in vacuum and at room temperature. I

After the two coating applications were completed in In applying the two coatings, it is Suchclose approximation however cannot 1, the diameter of the One ,of these factors is the orientation of the evaporator in a plane parallel to the faceplate 12. Such orientation should dispose the evaporator 16 'in coaxial relation with respect to the faceplate '12. An other factor is the spacing between the faceplate 12 and mately 300 C.. Applicant'found that during this heat treatment substantially all of the bismuth was converted into bismuth oxide, while the silver remained free from any noticeable oxidation.

After cooling, the baked subassembly It) Was exposed to air at room temperature and could be stored in air or incorporated immediately inpa .final, tube structure. Storage in air for as long as 36 hours has been found tolerable.

The final tube structure in the example under consideration, comprises the image tube shown in FIG. 2. The image tube referred to includes a second dish-shaped subassembly 30, also-made of glass, and having a flat portion 32 provided with a phosphor screen 34 thereon made of any conventional material and covered with a 'thin coating 36 of a material such as aluminum. The

subassembly 34) also includes a cylindrical flange 38 hav- 'ing an exhaust tubulation 44 extending therefrom. The

two sub'assemblies 10 and 31) are sealed together in a vacuum tight manner in a glass-to-gla-ss seal 42.

After evacuation of the envelope formed by the two subassemblies 10, and '30, through the exhaust tubulation 49, the tube is subjected-to the usual bake-out treatment at about 300 C. by which the tube components are heated to'a temperature sufficiently high to drive out occluded'gases therefrom. The bismuth-oxide layer 24 is next exposed to cesium from a source (not shown) within the tube envelope atv a temperature of about 150 C. until peak sensitivity of the photocathode comprising the silver and bismuth layer is reached.

Image tubes made in accordance with the foregoing method and in which the silver-bismuth oxide layersrhave been exposed to air for 36 hours, have shown sensitivities It is usually'not dein excess of 30 micro-amperes per lumen.

In FIG. 3 is shown an envelope subassembly 44 of an Image Orthicon type tube. .This subassembly is made of glass and includes a flat faceplate 46 having an inner surface torbe coated with silver and bismuth. The subassembly includes a' cylindrical flange 48. Evaporator 50 is similar to evaporator 16 shown in FIG. 1, and includes a wire 52, which may be made of tungsten, coiled for supporting and heating a pellet 54, which may be either silver or bismuth. a

The procedure in applying the silver coating 56 and the bismuth coating 58' is essentially the same as the procedure in applying the silvercoating 22 and bismuth coating 24, described in connection with FIG. 1.

After the silver coating 56 and the bismuth coating 58 have been appliedin vacuum, and the bismuth layerfully oxidized, in the manner described in connection with FIG. 1, the subassembly 44 may either be stored in air for immediately incorporated in' an Image Orthicon tube structure as shown in -FIG.. 4. Such incorporation involves sealing the subassembly 44 to a second subassembly fit also made of glass, in a glass-to-glass seal 62. After evacuation, the bismuth is exposed to cesium vapor from asource ,(not shown) withinthe tube envelope, at a P temperature of about 150 C: until peak sensitivity of the accordance with the foregoing, the coated subassembly 10'was exposed to oxygen at a pressure of one millimeter of mercury and heated therein for one hour at approxi The invention makes it feasible to, obtain a uniformly thick photocathode coating independently of the length of the tube envelope inwhich it is used, and at the same time secure image tubes and other conversion tubes containing'a photocathode having superior operating characteristics. 'In addition, the invention avoids-the need a for housing coating evaporators'within a tube envelope,

which is of advantage not only in tubes having relatively short envelopes, but also in tubes having relatively long envelopes. Another advantage of the invention is that it permits the photocathode and other elements of a tube to be processed simultaneously at separate locations, thereby saving time in fabrication.

What is claimed is:

1. Method of making an electron tube having a partly processed silver-bismuth-oxygen-cesium photocathode, comprising (a) applying to a substrate in vacuum and successively,

a layer of silver and a layer of bismuth,

(b) fully oxidizing said bismuth (c) and sealing said substrate across an opening in the envelope of said tube.

2. Method of making an electron tube having a partly processed silver-bismuth-oxygen-cesium photocathode, comprising:

(a) applying to a glass substrate in vacuum a coating of silver and a coating of bismuth on said coating of silver,

(b) heating said coated substrate in an oxidizing atmosphere to fully oxidize said coating of bismuth (c) exposing said substrate to the air,

(d) and fixing said substrate to the structure of said 1 tube. 3. in a method of making a silver-bismuth-oxygencesium photocathode, the steps of:

(a) coating a substrate successively in vacuum with layers of silver and bismuth,

(b) and heating the coated substrate in oxygen to fully oxidize said bismuth layer, whereby said photocathode may be exposed to the air with freedom from adverse effects.

4. Method of making an electron tube having a silverbismuth-oxygen-cesium photocathode, comprising:

(a) fabricating a portion of said photocathode in vacuum at one location,

(b) fabricating the remainder of said tube at another location,

(c) joining said portion of the photocathode to said remainder of said tube,

(d) evacuating said tube, and

(e) completing the fabrication of said photocathode in the evacuated tube.

5. Method of making an electron tube having a relatively short envelope, with a face plate at one end thereof, comprising:

(a) coating a surface of said face plate in Vacuum and at a location removed from the remainder of said envelope, with successive coatings of silver and bismuth, whereby said face plate may be spaced a desired distance from a coating source for contributing to uniformity in thickness of said coating,

(b) fully oxidizing said bismuth coating,

, (c) sealing said face plate across an opening in said envelope with said surface thereof facing inwardly t of said envelope,

(d) evacuating said envelope,

(e) and exposing said oxidized bismuth coating to cesium until peak sensitivity of said coating is reached.

6. A sub-assembly for an electron tube having a partly processed photocathode, comprising:

(a) a substrate having thereon a first coating of silver,

(b) and a second coating of bismuth oxide on said coating of silver.

7. Method of making a sub-assembly for an electron tube, comprising:

(a) coating in vacuum a substrate with a coating of silver having a thickness to transmit through said substrate and said silver coating about of light compared with transmission through said substrate alone,

(b) thereafter coating in vacuum and over said silver coating, a coating of bismuth to a thickness to transmit through said substrate and said silver and bismuth coatings about 55% of light compared with transmission through said substrate alone,

(c) and fully oxidizing said coating of bismuth to provide a partly processed photooathode free from adverse effect thereon of air.

8. In a method of making a photocathode including bismuth:

(a) the step of fully oxidizing said bismuth.

9. Method of making an electron tube, comprising the steps of:

(a) forming two dish-shaped glass members each having a cylindrical flange, the flanges of the two members having substantially the same diameter, whereby the free ends of said flanges are adapted to be placed in end butt relation,

(b) successively applying to the bottom surface of one or" said members in vacuum and with said one of said member spaced from the other of said members, coatingsof silver and bismuth,

(c) fully oxidizing said coating of bismuth,

(d) positioning said members to dispose said flanges with their free ends in end butt relation,

(e) mutually sealing said free ends, whereby said members form an enclosure,

(f) evacuating said enclosure, and

(g) activating said oxidized bismuth coating with cesium.

References Cited by the Examiner UNITED STATES PATENTS 9/59 Kerstelter 313-355 FRANK E. BAILEY, Primary Examiner.

UNITED STATES RATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,195,972 July 20, 1965 Alfred H. Sommer It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 22, for "photosensitive" read photoemissive column 3, line 38, for "accomplished" read accompanied Signed and sealed this 22nd day of February 1966.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents 

5. METHOD OF MAKING AN ELECTRON TUBE HAVING A RELATIVELY SHORT ENVELOPE, WITH A FACE PLATE AT ONE END THEREOF, COMPRISING: (A) COATING A SURFACE OF SAID FACE PLATE IN VACUUM AND AT A LOCATION REMOVED FROM THE REMAINDER OF SAID ENVELOPE, WITH SUCCESSIVE COATINGS OF SILVER AND BISMUTH, WHEREBY SAID FACE PLATE MAY BE SPACED A DESIRED DISTANCE FROM A COATING SOURCE FOR CONTRIBUTING TO UNIFORMITY IN THICKNESS OF SAID COATING, (B) FULLY OXIDIZING SAID BISMUTH COATING, (C) SEALING SAID FACE PLATE ACROSS AN OPENING IN SAID ENVELOPE WITH SAID SURFACE THEREOF FACING INWARDLY OF SAID ENVELOPE, (D) EVACUATING SAID ENVELOPE, (E) AND EXPOSING SAID OXIDIZED BISMUTH COATING TO CESIUM UNTIL PEAK SENSITIVITY OF SAID COATING IS REACHED. 